In regard to the active material for a negative electrode of a lithium ion secondary battery, metals, graphite and the like have been hitherto investigated as candidates. A metal electrode has a problem that when charging and discharging is repeated, lithium precipitates out in a dendrite form on the electrode and eventually causes a short circuit of both electrodes. For that reason, attention is being paid to a carbon material, particularly graphite, which has no precipitation of metallic lithium during the charge-discharge process.
Examples of using graphite as the active material for negative electrodes include natural graphite particles, artificial graphite particles obtained by graphitizing cokes, artificial graphite particles obtained by graphitizing an organic polymer material, pitch or the like, graphite particles obtained by pulverizing those graphite particles, and the like. These graphite particles are mixed with an organic binder and an organic solvent and formulated into a graphite paste, this graphite paste is applied on the surface of a copper foil, the solvent is dried, the dried paste is molded, and thereby, the product is used as a negative electrode for a lithium ion secondary battery. When graphite is used in the negative electrode, the problem of internal short-circuit occurring due to lithium dendrites is solved, and an improvement of the charge-discharge characteristics is promoted (see, for example, Patent Literature 1).
However, natural graphite in which graphite crystals have developed, has a weak binding force between the layers of crystals in the C-axis direction, as compared with the binding in the plane direction of crystals. Accordingly, the binding between graphite layers is broken by pulverization, and so-called flake-shaped graphite particles having a large aspect ratio are obtained. Since the flake-shaped graphite particles have a large aspect ratio, when the graphite particles are kneaded with an organic binder and applied on a collector to produce an electrode, the flake-shaped graphite particles are likely to orient in the plane direction of the collector. As a result, there is a problem that the charge-discharge characteristics or rapid charge-discharge characteristics are prone to deteriorate.
In order to address this problem, spheroidized graphite obtained by modifying flake-shaped graphite to approximate a spherical shape, has been proposed (see, for example, Patent Literature 2). The charge-discharge characteristics are enhanced by applying a mechanical treatment to flake-shaped graphite and thereby spheroidizing the particles.